1. Field of the Invention
The present invention relates to the field of electrolytic diaphragm cells and those which are particularly useful for the production of chlorine and caustic. The invention provides for a reduction in the cell voltage of the diaphragm cell.
2. Description of the Related Art
The diaphragm type electrolytic cell has found wide commercial use, such as for the electrolysis of brine to produce chlorine and caustic. The industry is constantly faced with the challenge of reducing operating expenses, including the cost of electric power. Efforts thus continue to be focused on increasing the efficiency of brine electrolysis.
On the one hand, it has been recognized that in operating these cells, it will be desirable to keep the anode-cathode gap small, since the resistance of the electrolyte in the gap contributes an ohmic over potential to the overall cell voltage. The cell voltage generally decreases linearly with the decrease of the anode-cathode gap. However, on the other hand, it has been stated that for anode-cathode distances below a limit of about 3.5-4 mm, the cell voltage remains more or less constant or may even increase (see Winings et al in Modern Chlor-Alkali Technology, 1980, pages 30-32).
Thus, it has been proposed to narrow this gap, as by moving the anode close to the diaphragm, but use an anode support member between the anode and the diaphragm to keep the anode from directly contacting the diaphragm. This approach has been taken in the innovation described in U.S. Pat. No. 3,242,059. As disclosed therein, a foraminous sheet of titanium is employed as an anode support. A side of this sheet is in contact with the diaphragm. An opposite side of the sheet has a coating, such as of platinum, that serves as the anode.
Along this same line, it has also been proposed to interpose a net between the anode and the diaphragm. This has been taught in U.S. Pat. No. 4,014,775. The net spaces the anode apart from the diaphragm by the thickness of the net, i.e., the spacing between the anode and the cathode is comprised of the thickness of the diaphragm plus that of the net.
In the continuing development of anode technology, the innovation of the expandable anode has experienced great commercial success. Expandable anodes have been described, for example, in U.S. Pat. No. 3,674,676. These expandable anodes have a shape somewhat like a hollow cereal box, i.e., minus its top and bottom, and may be referred to herein as expandable anodes. The anode surfaces can be kept in a contracted position, by the use of retainers, while the anode is inserted between cathodes. By removing the retainers, the anode surfaces are released and moved toward the surface of the diaphragms, which diaphragms may be deposited on the cathode.
Along with the development of the expandable anode, efforts continued toward the development of diaphragms having desirable stability in extended cell operation, and which diaphragms can serve while in direct contact with the anode surface. This is referred to as zero gap operation. Such zero gap operation has been achievable with an improved, resin reinforced, asbestos diaphragm such as disclosed in U.S. Pat. No. 4,563,260.
Even for these anodes of the expandable type, it has been found to be useful to place between the diaphragm and the anode an additional member. Thus, there has been disclosed, in European patent application No. 0 611 836 A1, the placement of a thin, expanded mesh on an anode that is a coarse mesh anode. By extending the expandable anode and this fine mesh surface against the diaphragm, the diaphragm retains its original thickness and does not undergo any volume expansion. Also, in this dual structure, the pressure exerted may pressingly increase fiber cohesion and thereby enhance fiber retention in the diaphragm during cell operation.
It would nevertheless be desirable to further improve the operating efficiency of a diaphragm cell. It would be particularly desirable to improve such efficiency by a further reduction in the cell voltage. It would also be desirable to achieve this additional operating efficiency while at the same time obviating any problem of a constant cell voltage, or even increase in cell voltage, that can be encountered as the anode-cathode distance is decreased below a limit of about 3-4 mm.
A reduction in cell voltage of a diaphragm cell has now been achieved. This has been obtained without occasioning any problems associated with a more or less constant cell voltage, or even cell voltage increase, as can be encountered as the anode-cathode distance is reduced below a certain limit. The present invention is suitable for utilization with structures including expandable anodes which may or may not include a fine or compressible material, as an intermediate element, located between the anode and the diaphragm. The invention can be serviceable in such structures where an intermediate element, if present, may or may not be coated. The invention resides generally in the discovery that cell operation that is not only non-detrimental, but is also enhanced, can be achieved by permitting compression of the diaphragm. This compression provides for cell operation that can be described as less than zero gap operation. The innovation achieves a highly desirable reduction in the cell voltage of the diaphragm cell. This reduction in cell voltage can be obtained so that a measurable lower overall power consumption can be achieved.
In one aspect, the invention pertains to an electrolytic diaphragm cell having a diaphragm interposed between electrodes of the cell, such cell comprising an anode assembly having at least one anode contacting the diaphragm and a cathode assembly having at least one cathode contacting the diaphragm, with the anode and the cathode providing an interelectrode gap, which interelectrode gap contains the diaphragm, with the diaphragm having an original and uncompressed thickness within the electrode gap as a first thickness, the improvement in such cell comprising a diaphragm compressed by pressing at least one electrode against the diaphragm, which diaphragm is present in the interelectrode gap as a compressed diaphragm of a econd, reduced thickness.
In another aspect, the invention is directed to a method for assembling an electrolytic diaphragm cell for the electrolysis of an aqueous electrolyte, which method comprises:
establishing a metal anode;
providing a metal cathode adjacent the anode, with the cell having an interelectrode gap between the anode and the cathode;
establishing a diaphragm of a first, original and uncompressed thickness within the interelectrode gap between the anode and the cathode; and
pressing at least one electrode against the diaphragm and compressing the diaphragm to a second, reduced thickness.
In a still further aspect, the invention is directed to the process wherein an electrolyte is passed into an electrolytic cell and electrolyzed in the cell, and the cell contains a compressible diaphragm positioned between the anode and the cathode, which diaphragm is placed in the cell in a first original and uncompressed thickness, the improvement in the process which comprises electrolyzing the electrolyte in the cell with the cell containing the diaphragm compressed between the anode and the cathode, which diaphragm is compressed to a second, reduced thickness.